After considerable refinement of the micro-MLC technique, we have substantiated several preliminary observations regarding the use of this technique for use in monitoring the immunologic status of recipients of renal allografts. The MLC technique can be used to predict clinical rejection episodes in HLA identical pairs, neither pretransplant MLC reactivity nor posttransplant inhibition of the MLC response in recipient plasma is predictive of rejection in non HLA identical pairs. However, the loss of recipient cell response in the immediate posttransplant period may be predictive of future rejection episodes.